[Suicide Junkie]

Do the ships need to have capture planet as their design orders?
As soon as you break formation your ships start using their individual strategies.

Also, if the planet has weapons, the ships trying to capture will hang back until the armed ships are destroyed or the planet is disarmed. At which point they will charge in.

All that is why I removed weapon platforms from my mod and replaced them with orbital defenses... since the planet is unarmed (though very well armored against weapons fire with troops in most cases) the troop transports rush in. And since there are armed hostiles in orbit, the warships and fighters rush in too and provide cover fire. And the troops land in all the confusion.